Research On The Key Technologies Of Operation And Maintenance Of HVDC Cable

HVDC cable is an important part of HVDC transmission system.The research on the key technologies of HVDC cable operation and maintenance is of great significance to the popularization of HVDC transmission,so as to reduce the maintenance cost and improve the economy of HVDC transmission.The electric field distribution and the safe operation of the cable are affected by the conductor temperature during the operation of HVDC cable.Studying the electric field distribution of cable with defects is helpful for us to take corresponding preventive measures to avoid defects in cable design,manufacture and installation.Thermal aging test of cable insulation material can predict the life span of the cable and evaluate the status of the cable for the purpose of formulating maintenance strategies.In this paper,±500kV HVDC XLPE cable was taken as the research object,and the key technologies of operation and maintenance of the cable were studied.First,the operating temperature of the cable was studied.Based on the Comsol finite element software,a two-dimensional model was established to simulate the temperature field of the ±500kV HVDC XLPE cable in a steady state,and the steady-state current carrying capacity was calculated.According to the conductor temperature suitable for the long-term operation of the cable,the current carrying capacity of the normal operation of the cable was calculated;it is concluded that the current carrying capacity has a large difference between the conductor temperature and the temperature of the insulation layer by drawing the three-dimensional diagram;the safety range of the normal operation of the cable was planned,when the ambient temperature is less than 15℃ the operating temperature is affected by the temperature difference of the insulating layer,and the operating temperature is affected by the ampacity when the ambient temperature is greater than 15°C.Secondly,a load cycle test was carried out to measure the temperature of the outer sheath,the insulation shielding layer and the conductor.The temperature of conductor and insulation shielding layer rise smoothly.Because the outer sheath contacts with the air directly,the temperature of the outer sheath rises tortuously;the temperature field was subjected to transient simulation,and the error between the simulation results and the test data is less than the specified value,which verifies the correctness of the transient simulation,and then the transient and steady-state results were compared to verify the correctness of the steady-state simulation.Thirdly,the electric field simulation model of the cable was established.The simulation results show that as the current carrying capacity increases,the electric field distribution gradually reverses,and the electric field distribution in the insulating layer rotates counterclockwise around the midpoint of the insulating layer;there is a simulated core cable defects such as metal burrs,residual air gaps in the insulation,and scratches on the insulation layer.The results show that the first type of defect has the greatest impact on the cable,and the third type of defect has the least impact.Finally,a thermal aging test was performed on the insulating material XLPE sample.After thermal aging,the sample turned yellow and brittle,and the insulation performance decreased seriously;with the increase of aging time and aging temperature,the elongation at break of the samples decreases seriously,and the mechanical properties severe degradation;the thermal aging model of XLPE cable was established by using the treated data.The volume resistivity,dielectric constant and dielectric loss tanδ of the sample were measured.The results show that the volume resistivity decreases during aging,while the dielectric constant and tanδ increase during aging,and the change of frequency has little effect on them.